In man, respiratory syncytial (RS) virus is responsible for the majority of respiratory illnesses suffered by children during the first five years of life. Symptoms are those of coryza, bronchitis and bronchopneumonia and may last from days to several weeks. While recovery is the usual outcome, morbidity is often quite significant and may necessitate aggressive symptomatic clinical treatment including mechanical respiratory assistance. Epidemiologic studies also suggest the possibility of a causal relationship between overt RS virus infection and the development of asthma later in childhood. Despite recognition of the clinical and economic importance of RS virus infections, neither preventive nor suppressive treatment is available. Attempts to provide active vaccination have met with failure, and as yet no effective anti-viral drug has been found. This state of affairs has led to the present invention providing for new avenues to the control of the virus.
RS virus has been grouped as a member of the genus pneumovirus within the family of paramyxoviridae. Like the other two genera in this family, i.e., the paramyxoviruses and the morbilliviruses, it is characterized by a helical arrangement of the RNA-containing nucleocapsid, by the possession of an outer envelope bearing numerous projections ("spikes") and by its ability to produce cell fusion in tissue cultures. For at least three members of the genus paramyxovirus, i.e., Sendai virus, 6/94 virus, and Newcastle disease virus, a trypsin-like proteolytic activity is a necessary factor in achieving full expression of the biologic properties of the virions (Scheid and Choppin, Identification of biological activities of paramyxovirus glycoproteins. Activation of cell fusion, hemolysis, and infectivity by proteolytic cleavage of an inactive precursor protein of Sendai virus. Virology 57, 475-490 (1974); Waters et al., The potential role of phagocytically active cells in establishing chronic parainfluenza 1 virus infections in human brain J. Neur. Sci. 25, 491-498 (1975); Nagai and Klenk, Activation of precursors to both glycoproteins of Newcastle disease virus by proteolytic cleavage. Virology 77, 125-134 (1977). It has been clearly demonstrated that the action of the putative enzyme involves the post-translational cleavage of a virion glycoprotein which is changed from an inactive precursor into an active fusion (F) protein. The F protein plays a central role in the fusion of virions with cell membranes, in the formation of the characteristic syncytia in cell cultures and also in the virus-dependent lysis of red cells (Scheid and Choppin, supra. Biologically, the factor controls infectivity and virulence of the viruses and the severity of the cytopathic effect.
The general similarities in both the morphologic and cytopathogenic characteristics of these three viruses and RS virus suggested to us the possibility that a protease might also participate in the replication of the latter agents. If so, suppression of the cleavage event could be expected to interfere with the cytotoxic activities and with the replication of the viruses (Korant, Proteolytic events in viral replication. In Molecular Basis of Biological Degradative Processes. (R. D. Berlin, H. Herrmann, I. H. Lepow, and J. M. Tanzer, eds.), pp. 171-224, Academic Press, New York, 1978). To this end, RS virus was cultured in the presence of a series of reversible inhibitors of arginine-directed (i.e., trypsin-like) esteroproteases. The investigation led to the discovery of seven compounds which strongly blocked RS virus-induced cell fusion. It should be noted, that inhibitory potency per se does not allow any judgment as to the mode of action of the compounds. Inhibition of posttranslational hydrolytic cleavage of virus proteins, may play a role, but may not be the only, or even the major factor involved.
Despite recognition of the clinical and economic importance of RS virus infections, neither effective preventive nor suppressive treatment is available. Attempts to provide active vaccination have met with failure, and as yet no effective antiviral drug has been found. This state of affairs has led to the present invention providing for new avenues to the control of the virus.
Accordingly, it is a primary object of the present invention to provide a method for inhibiting Respiratory Syncytial Virus-Induced Cell Fusion.
This and other objects of the present invention will be more apparent from the discussion below.